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newSchoolWorks.com › VoTechWorks › Agriculture, Environment & Public Service › Water / Wastewater Operator › Water Chemistry for Operators › Lesson 6
AGR-WATE71-02 · Lesson 6 · 14.5 clock hours · theory

Chlorine Chemistry and Disinfection

Objective. Students will explain free chlorine, combined chlorine, and total chlorine chemistry, describe breakpoint chlorination, and calculate chlorine dose, demand, and residual.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Why Chlorine Is Used in Water Treatment

Chlorine is the most widely used disinfectant in drinking water and wastewater treatment in the United States. It kills or inactivates bacteria, viruses, and protozoa, and it maintains a residual in the distribution system that protects against recontamination. Understanding the chemistry behind chlorine is essential for every licensed operator.

Forms of Chlorine in Water

When chlorine gas (Cl₂) or sodium hypochlorite (NaOCl) is added to water, it reacts to form hypochlorous acid (HOCl) and the hypochlorite ion (OCl⁻). Together these two species are called free chlorine (also written free available chlorine, or FAC).

The balance between HOCl and OCl⁻ depends almost entirely on pH:

This matters because HOCl is the far more powerful disinfectant — roughly 80 times more effective at killing pathogens than OCl⁻. Keeping pH in the range of 6.5–7.5 maximizes disinfecting power while staying within typical drinking water standards.

When free chlorine reacts with ammonia (naturally present in some source waters, or added intentionally), it forms chloramines, which are collectively called combined chlorine. The three forms are monochloramine (NH₂Cl), dichloramine (NHCl₂), and nitrogen trichloride (NCl₃). Chloramines are weaker disinfectants than free chlorine but produce fewer disinfection byproducts (DBPs) and maintain a more stable residual over long distribution distances.

Total chlorine = Free chlorine + Combined chlorine.

Operators measure all three with a colorimetric test (commonly the DPD method) or amperometric titration.

Chlorine Dose, Demand, and Residual

Three related terms appear on every state licensing exam:

The fundamental relationship is:

> Dose = Demand + Residual

Rearranged: Residual = Dose − Demand or Demand = Dose − Residual

Knowing any two values lets you calculate the third.

Breakpoint Chlorination

Breakpoint chlorination describes what happens as the chlorine dose is gradually increased in water that contains ammonia.

1. Phase 1 (below breakpoint): Added chlorine reacts with ammonia to form chloramines. Total chlorine rises as combined chlorine increases; free chlorine is essentially zero.

2. Phase 2 (approaching breakpoint): Further addition of chlorine oxidizes the chloramines, destroying them. The measured total chlorine residual actually decreases even as more chlorine is added. Odorous compounds such as dichloramine and nitrogen trichloride may form in this zone.

3. Breakpoint: The point at which chloramines have been completely destroyed. The residual hits its minimum.

4. Phase 4 (beyond breakpoint): Any additional chlorine appears as free chlorine residual on a near 1:1 basis.

Reaching breakpoint typically requires a chlorine-to-ammonia-nitrogen ratio of approximately 10:1 by weight, though the exact ratio depends on water chemistry. Operating past the breakpoint is how plants achieve a free chlorine residual in the distribution system.

CT Concept for Pathogen Inactivation

CT stands for Concentration × Time (mg/L · minutes). Regulatory agencies use CT values to verify that a disinfection process achieves a required log inactivation of target pathogens such as Giardia and viruses. A higher CT means either a higher residual concentration held for a shorter contact time, or a lower concentration held longer. Temperature and pH affect CT requirements — colder water and higher pH both require a greater CT to achieve the same level of inactivation. Operators must document CT daily to demonstrate compliance with disinfection rules.

Safety Points

Chlorine gas is extremely hazardous — it is a NIOSH-classified immediately dangerous to life or health (IDLH) substance. Always:

Always consult your facility's Safety Data Sheets (SDS) and follow OSHA and EPA requirements for chlorine handling.

Key terms9

Free chlorineThe combined concentration of hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻) present in water; the primary active disinfecting form of chlorine.
Combined chlorineChlorine that has reacted with ammonia to form chloramines (monochloramine, dichloramine, nitrogen trichloride); a weaker disinfectant than free chlorine.
Total chlorineThe sum of free chlorine and combined chlorine measured in a water sample.
Chlorine demandThe amount of chlorine consumed by reactions with organic matter, ammonia, metals, and other substances in the water before a residual is established.
Chlorine residualThe measurable chlorine remaining in water after the chlorine demand has been satisfied; required in distribution systems to prevent recontamination.
Breakpoint chlorinationThe point in chlorination at which chloramines have been fully oxidized and destroyed; adding chlorine beyond this point produces a free chlorine residual.
Hypochlorous acid (HOCl)The undissociated, neutral form of free chlorine; the most effective disinfecting species, dominant at lower pH values.
CT valueThe product of disinfectant concentration (mg/L) and contact time (minutes) used to assess whether a disinfection process achieves required pathogen inactivation.
DPD methodA common colorimetric test using N,N-diethyl-p-phenylenediamine reagent to measure free, combined, and total chlorine residuals in water.

Worked examples4

1. A water sample is tested and the chlorine demand is found to be 1.8 mg/L. The target free chlorine residual in the distribution system is 0.5 mg/L. What chlorine dose (mg/L) must the operator apply?
  1. Write the relationship: Dose = Demand + Residual
  2. Substitute the known values: Dose = 1.8 mg/L + 0.5 mg/L
  3. Calculate: Dose = 2.3 mg/L
Answer: The required chlorine dose is 2.3 mg/L.
2. An operator adds a chlorine dose of 3.2 mg/L to a water sample. After adequate contact time, the measured total chlorine residual is 0.7 mg/L. What is the chlorine demand of this water?
  1. Write the relationship: Demand = Dose − Residual
  2. Substitute the known values: Demand = 3.2 mg/L − 0.7 mg/L
  3. Calculate: Demand = 2.5 mg/L
Answer: The chlorine demand is 2.5 mg/L.
3. A plant treats 2.5 million gallons per day (MGD). The required chlorine dose is 2.0 mg/L. How many pounds of chlorine must be fed per day? (Use the water treatment conversion: lb/day = flow (MGD) × dose (mg/L) × 8.34 lb/gal)
  1. Write the pounds-per-day formula: lb/day = Flow (MGD) × Dose (mg/L) × 8.34
  2. Substitute values: lb/day = 2.5 × 2.0 × 8.34
  3. Multiply step by step: 2.5 × 2.0 = 5.0; then 5.0 × 8.34 = 41.7
  4. Result: 41.7 lb/day
Answer: The plant must feed approximately 41.7 pounds of chlorine per day.
4. After testing, a water sample shows a total chlorine residual of 1.4 mg/L and a free chlorine residual of 0.9 mg/L. What is the combined chlorine concentration?
  1. Write the relationship: Combined chlorine = Total chlorine − Free chlorine
  2. Substitute the known values: Combined chlorine = 1.4 mg/L − 0.9 mg/L
  3. Calculate: Combined chlorine = 0.5 mg/L
Answer: The combined chlorine concentration is 0.5 mg/L.

The math in this lesson1

Learners compute chlorine dose = demand + residual, 1.8 + 0.5 = 2.3 mg/L.
7.EE.3
In class: Illustrated lecture: chlorine forms in water (HOCl, OCl⁻, chloramines), the effect of pH on disinfecting power, breakpoint chlorination curve, and CT concept for pathogen inactivation; students annotate a breakpoint curve diagram. · Worked calculation series: instructor models three dose/demand/residual problems on the board (e.g., find required dose to achieve a target residual given measured demand); students independently solve three similar problems and peer-check answers. · Scenario discussion: a distribution system shows zero chlorine residual at a dead-end main; small groups identify at least three possible causes and one corrective action each, then share with the class.
Materials: Projector, breakpoint chlorination curve handout, calculation worksheet, AWWA Chlorination/Chloramination reference or equivalent operator manual, student notebooks.
Check: Calculation worksheet: students correctly solve at least 4 of 5 dose/demand/residual problems within 5% of correct answer (80% pass mark); annotated breakpoint diagram reviewed by instructor for accuracy.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Water Treatment — Disinfection
1. Which form of chlorine is the most effective disinfectant in water treatment?
Water Treatment — Disinfection
2. As the pH of a water sample rises from 6.5 to 9.0, what happens to the disinfecting power of a given free chlorine residual?
Water Treatment — Calculations
3. A water sample has a chlorine demand of 2.4 mg/L. An operator wants to maintain a residual of 0.6 mg/L. What dose is required?
Water Treatment — Disinfection
4. On a breakpoint chlorination curve, what occurs in the zone just before the breakpoint is reached?
Water Treatment — Calculations
5. A plant doses 4.5 mg/L of chlorine and measures a residual of 1.1 mg/L. What is the chlorine demand?
Water Treatment — Regulatory Compliance
6. What does the CT value represent in disinfection practice?
Water Treatment — Calculations
7. A water system is treating 1.5 MGD and requires a chlorine dose of 2.8 mg/L. Using the factor 8.34 lb/gal, approximately how many pounds of chlorine must be applied each day?
Operator Health and Safety
8. When handling chlorine gas cylinders, which personal protective equipment (PPE) combination is most appropriate for responding to a suspected leak?
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